This invention relates to a feedback type ignition timing control system for internal combustion engines which advances or retards the ignition timing in response to a signal corresponding to the torque or combustion phenomenon such as knocking phenomenon of an engine.
The ignition timing of an engine must be controlled according to the engine conditions so as to ensure the optimum operation of the engine.
Ignition timing control systems known in the art are generally so designed that the rotational speed and intake vacuum of an engine are detected to represent the condition of the engine and the ignition timing is controlled in accordance with the detected engine condition.
It is known in the art that in consideration of the efficiency and fuel consumption of an engine, the best way is to effect the ignition at around a so-called MBT (the minimum advance for best torque) and the ignition timing must be adjusted to MBT in accordance with the engine conditions.
However, this MBT is affected not only by the above-mentioned factors, i.e., the engine speed and load but also by such factors as the supplied air-fuel mixture ratio, the amount of exhaust gas recirculation (EGR), the engine temperature and the atmospheric condition and consequently it is extremely difficult to effect the ignition at MBT under all the operating conditions of the engine. Further, under certain engine operating conditions knock will be produced later than MBT and it is necessary to prevent the occurrence of knock. As a result, in the case of the known systems the ignition timing is set considerably after MBT so as to prevent knocking. This restrains the power output and fuel consumption of an engine below its performance. With a view to overcoming these deficiencies, feedback type ignition timing control systems have been proposed in which is detected the acceleration, speed or the like associated with the pressure in the combustion chamber of an engine or the engine vibration produced externally by such pressure factor, whereby the torque is calculated or the detection of knocking phenomenon is effected and feedback is applied to control the ignition timing in accordance with the detection signal so as to advance or retard the ignition timing. In this type of feedback system, generally the operation of addition or subtraction is performed on a base advance angle value .theta..sub.M and a feedback correction value .theta..sub.K and the resulting .theta..sub.M +.theta..sub.K or .theta..sub.M -.theta..sub.K is utilized to control the ignition timing. In this case, the usual method of determining the base advance angle value .theta..sub.M is to compute the same in accordance with such main engine parameters as the amount of air flow, rotational speed and intake pressure as well as such compensating parameters as warm-up condition, amount of EGR and acceleration/deceleration operation or to read out the value from a preliminarily programmed fixed map. On the other hand, irrespective of the main engine parameters, the feedback correction value .theta..sub.K is produced only from the output signal of a feedback factor detector for detecting a knocking phenomenon, combustion pressure or the like and it is used to correct the value .theta..sub.M so as to generally shift the same and thereby to effect the ignition.
However, this type of known system has the following disadvantages. Firstly, due to a fairly wide range of differences in the combustion of an engine, if the correction value is computed statistically on the basis of the data obtained through the ignition cycles ranging from several tens to several hundreds so as to reduce the differences in combustion, under transient conditions a delay will be caused in the system's response and the follow-up or response quality will be deteriorated.
Another disadvantage is that if the correction value .theta..sub.K is computed for every ignition cycle instead of computing it through a statistical procedure, the range of hunting will be increased extremely by the variations in combustion so that the combustion will be varied greatly and thus the torque will be varied, giving rise to a detrimental effect on the drivability. If the amount of correction for each ignition cycle is reduced so as to reduce the range of hunting, the delay in response will be increased as is the case when the correction value is computed statistically.